Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a control method thereof.
Description of the Related Art
As an exposure method adopted by an exposing unit of an electrophotographic image forming apparatus, there is a laser exposure method. In a laser exposure method, there is used a lens for guiding a beam of laser light from a light source unit to a scanning unit and for causing the beam of the laser light, which is deflected and scanned by the scanning unit, to form an image on a photosensitive member. It is desirable for the scanning speed of the laser light that scans the surface of the photosensitive member to be constant regardless of the position of the laser light on the surface of the photosensitive member. It is also desirable for the size (to be referred to as a spot diameter hereinafter) of a spot shape which is formed on the surface of the photosensitive member to be uniform regardless of the position of the spot shape on the surface of the photosensitive member. Hence, a lens that has an fθ characteristic is generally used as the image forming lens. By using a lens that has an fθ characteristic as the image forming lens, the scanning speed of the laser light that scans the surface of the photosensitive member will be constant regardless of the position of the laser light on the surface of the photosensitive member, and the size (to be referred to as the spot diameter hereinafter) of each spot shape formed on the surface of the photosensitive member will be uniform regardless of the position of the spot on the surface of the photosensitive member.
On the other hand, there is an example of a design in which an image forming lens that does not have an fθ characteristic is used for the purposes of downsizing and cost reduction. The scanning speed will not be constant and the spot diameter will not be uniform when an image forming lens that does not have the fθ characteristic is used. In Japanese Patent Laid-Open No. 2016-000511, there is disclosed a method of correcting the emission luminance of laser light so that the exposure amount per unit area on the surface of a drum will be constant without using an fθ-characteristic lens.
However, even in a case in which the emission luminance of the laser light is adjusted so that the exposure amount per unit area on the surface of the drum will be constant, the line widths will not be uniform since each spot diameter will differ depending on its position in the main scanning direction.